In producing continuous glass filaments, it is conventional to flow individual streams of molten glass from closely spaced orifices in the bottom wall of an electrically heated platinum or platinum alloy bushing. Apparatus associated with the bushing draws or attenuates the individual streams into filaments, which may be either continuous or discontinuous.
There are two types of continuous filament bushings in general use. A first, conventional, more widely-used type employs a bottom wall or orifice plate having stream-emitting orifices provided with tubular tips projecting from its undersurface, while a second, more recently developed type utilizes a plane bottom wall having a flat exterior surface. The first type is known as a "tipped" bushing, and examples are disclosed in Russell U.S. Pat. No. Re. 24,060 and in Glaser et al U.S. Pat. Nos. 4,222,757 and 4,321,074. The second type of bushing is known as a "tipless" bushing, and an example of such a bushing is disclosed in Strickland U.S. Pat. No. 3,905,790.
In both types of forming apparatus, the glass fiber is attenuated from a forming "cone" at the exit end of the forming orifice. The "cone" is of a smooth asymptotic shape from an upper base, which is essentially the diameter of the orifice, to the attenuated fiber. The head pressure of the molten glass in the bushing is reduced to the desired glass pressure in the cone by the pressure drop across the orifice, and in the case of a "tipped" bushing, by the pressure drop across the tip.
Upon interruption of the fiber attenuation process at any given orifice, molten glass flows from the orifice under the bushing head pressure to form a "bead" which drops into the "veil" of filaments attenuated from the other orifices. This bead interrupts filament production, requiring a fresh start-up of the complete bushing. The resultant intermittent operation substantially reduces the operating efficiency of the entire operation.
It would be desirable to prevent the formation and dropping of the beads upon fiber interruption and to avoid the necessity of manual start-up of the complete bushing upon bead formation and drop.